Skeletal muscle is a highly regenerative tissue in which growth, maintenance, and repair depends on the activity of a self-renewing stem cell population. A variety of cells identified by a variety of means from adult skeletal muscle have been ascribed regenerative potential. Our understanding of the precise identity of the muscle stem cell and its characteristics is severely hampered by a lack of animal models to identify, track, and manipulate gene expression in the muscle stem cell, a situation that has led to controversies in the field. The myogenic master regulator, Pax7, is expressed in a highly selective manner in muscle stem cells, and is absolutely and non-redundantly required for their maintenance and self-renewal in adult skeletal muscle. We have developed and validated a BAC transgenic strategy to drive gene expression specifically in muscle stem cells by using a very large regulatory domain of the Pax7 gene. We request support to develop genetic tools based on this strategy that will allow the precise study of the adult skeletal muscle stem cell, with a specific focus on addressing how the physiology and regenerative potential of this cell changes in aged mice, both intrinsically and in response to environmental variables such as acute injury or lifelong voluntary exercise. PUBLIC HEALTH RELEVANCE: Skeletal muscles of adult mammals contain a resident population of stem cells with the ability to regenerate after muscle injury, and required for the maintenance of muscle throughout adult life. Much of our current understanding of the muscle stem cell is based on studying cells in muscle sections, or studying cells extracted from muscle and grown in vitro. Current technology for purifying stem cells by cell sorting from muscle cell preparations is problematic as there is no single marker that distinguishes muscle stem cells from other cell types resident in muscle. We have developed a method, based on manipulation of the Pax7 gene, which allows us to uniquely identify muscle stem cells and isolate them by flow cytometry (cell sorting). We propose to use this technology to develop mouse strains suitable to the study of how the aging process affects the proliferation, differentiation, and muscle regeneration capacity of the muscle stem cell and to determine how these changes are impacted by voluntary exercise.